fixed visibility toggle with node selection

before this PR toggling visibility of not selected nodes would toggle
the visibility of the whole selection.
This PR changes this behaviour, so if visibility is toggled on a node
that it is not selected, the visibility toggle will happen only on this
node.
No changes to the behaviour of when one of the selected nodes has
visibility toggled: they still toggle all together

editor/scene_tree_editor.cpp

@@ -73,7 +73,7 @@ void SceneTreeEditor::_cell_button_pressed(Object *p_item, int p_column, int p_i
 		undo_redo->create_action(TTR("Toggle Visible"));
 		_toggle_visible(n);
 		List<Node *> selection = editor_selection->get_selected_node_list();
-		if (selection.size() > 1) {
+		if (selection.size() > 1 && selection.find(n) != NULL) {
 			for (List<Node *>::Element *E = selection.front(); E; E = E->next()) {
 				Node *nv = E->get();
 				ERR_FAIL_COND(!nv);